The present invention relates to a semiconductor integrated circuit device (hereinafter referred to as IC) and, more specifically deals with an IC in which is formed an amplifier circuit.
In an IC having elements that are formed on a substrate, it is necessary that the substrate is fixed to ground potential so that the substrate is separated from the elements by the pn junction.
On the other hand, reference potential wiring (earth) of the circuit formed in the IC must be fixed to ground potential by providing on the substrate a wire bonding pad for grounding the circuit, and connecting the pad to an externally drawing lead of the IC (hereinafter often referred to as external connecting pin of the IC) using a metal connector.
In general, the number of external connecting pins of the IC is determined depending upon the package construction of IC. It is desired to minimize the number of external connecting pins (external connecting terminals) in the circuit which is produced in an integrated circuit form. From such a point of view, the wire bonding pad for grounding the circuit and the wire bonding pad for grounding the substrate had hitherto been formed on the substrate in the form of a common wire bonding pad, and the common pad had been connected to an external connecting pin of the IC via a metal connector so as to be maintained at ground potential of an electronic device which consists of a printed board or the like.
However, the inventors of the present invention have found the fact that when the IC is employed for an audio amplifier circuit, the distortion factor of the audio amplifier circuit is deteriorated by the internal resistance possessed by the metal connector which connects the common wire bonding pad to the external connecting pin. In other words, since a heavy current such as output current flows from a reference potential to ground potential in the circuit which is constructed in the form of an integrated circuit, the reference potential undergoes variation in the IC due to internal resistance possessed by the metal connector, whereby the potential of the substrate is also varied. Variation in the substrate potential caused by the output current deteriorates the distortion factor of the output in the audio amplifier circuit.
If illustrated in further detail in regard to the IC which contains an audio amplifier circuit having at least an output amplifier circuit portion and an input amplifier circuit portion, a reference potential line in the output amplifier circuit portion is separated from a reference potential line in the input amplifier circuit portion in the IC in order that the input side is not affected by the variation in the reference potential caused by a heavy current in the output amplifier circuit portion, and wire bonding pads and externally drawing leads are provided to individually ground them.
In this case, when the substrate is connected to the wire bonding pad for grounding in the output amplifier circuit, the output current flows through the metal connector on the output side and through the externally drawing lead. However, since the metal connector contains internal resistance, the reference potential line in the output amplifier circuit undergoes variation due to output current, and the substrate potential undergoes the variation as well. Variation in the substrate potential is fed back by a parasitic capacity which is formed between a region where the input-stage transistor is formed and the substrate in the above-mentioned region, and the thus fed-back signals are superposed on the input signals to deteriorate the distortion factor.
On the other hand, when the substrate is connected to the wire bonding pad for grounding in the input amplifier circuit, the leakage current flows into the substrate via a pnp lateral transistor which forms a generally employed quasicomplementary push-pull circuit. The leakage current flows through the metal connector on the input side and through the externally drawing lead. Here, since the metal connector contains resistance, the reference potential line in the input amplifier circuit portion is varied by the leakage current which flows into the substrate, and the distortion factor is deteriorated.